This invention relates to a tuning voltage supply and automatic fine tuning correction circuitry for color television receivers employing varactor tuners.
Certain state of the art color television receivers incorporate a controller which comprises a resistive element with switch selected taps for selecting a desired channel by varying the controller voltage thereby setting the tuner voltage. This voltage is applied to the varactor tuner and controls the oscillator frequency. The controller is normally operated by a single knob positioned on the front panel of the receiver such that an operator can readily select the desired channel by adjustment of the knob. While the material, which serves as the resistive element of certain controllers, has desired characteristics in conjunction with varactor tuners, the resistive material is sensitive to temperature changes, humidity and other ambient conditions. Slight changes or fluctuations in the resistance of the material cause concomitant changes in the tuner voltage with a resultant drift in the frequency of the signal received and amplified. Accordingly, the quality of the reproduction of the scene telecast by the station may be diminished or the channel received may not correspond with the dial channel indication.
In order for a controller to function properly, it is necessary for a preselected source voltage to be applied to it such that the output of the controller, the tuner voltage, is predetermined. Otherwise, the tuner voltage will drift and cause a spurious change in the oscillator frequency. Moreover, the tuner operation is enhanced if a preselected correction voltage is superimposed on the automatic fine tuner voltage fed into the tuner.
Accordingly, it is an object of this invention to provide an improved tuning system for a color television receiver embodying a varactor tuner having a controller which provides a preselected tuning voltage. Another object of the invention is to provide an improved tuning system which includes a tuning voltage supply that feeds a preselected source voltage to the controller. A further object of the invention is to provide a tuning voltage supply having a low impedance drive for applying a preselected source voltage to the controller such that the source voltage is substantially independent of the resistive value of the controller. Yet another object of the invention is to provide an automatic fine tuning interface combined with a low impedance tuning voltage supply such that the automatic fine tuning interface supplies a correction voltage to the varactor tuner to compensate for tuner oscillator frequency errors due to temperature drift or customer misadjustment. Other objects and advantages of the invention will become apparent upon reading the following specification together with the drawings in which: